Bacteria That Feast on Dead Human Skin Live Inside Air Conditioners And Make Our Rooms Like Pee

Air conditioners are a host to an array of bacteria and fungal species. Spencer Platt/Getty Images

The mystery of foul-smelling air conditioners has finally been identified and it appears that the culprit is not dust or dirt but rather our own skin cells. According a new study from Hong Kong, tiny bacteria in air conditioning systems feeding on our skin produces an ammonia scent.

The new study isn't the first to examine the link between dead human flesh and air conditioner odors. Prior work has hinted that the mechanism responsible for the smell is the consumption of our sloughed-off skin cells by bacteria.

A past study identified two airborne species of bacteria: Staphylococcus haemolyticus and Methylobacterium organophilum, which account for the majority of microbial species found in air conditioning units, whether or not they give off odors.

That same study pointed to the break down of keratin, a protein in our skin, by the bacteria as the underlying cause of the smell: A/C units that smelled bad had more keratinase activity, the name for that break-down process. Still, the evidence didn't prove the connection and the role our skin plays in making our homes smell like urine remained murky.

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The new study, published in Indoor Air, examined seven bacterial species and four fungal species commonly found in both air conditioning units and single-room air conditioners. Results confirmed that the keratin consumed in the bacterial meal of skin flakes (also called squames) shed from the human body were the main source of the smell. That feast produces ammonium, a chemical that smells like urine.

In addition to keratin, other compounds found on skin cells, such as lipids and fatty acids, can also produce foul-smelling odors when broken down by the bacteria found in these air conditioning units. The study also explains why air conditioner smells may persist even in environments free from other forms of dust.

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The reason why ammonia reminds us of urine is because urea, the main waste product found in urine, can be further broken down into down into ammonia in certain situations, such as extreme dehydration or having a urinary tract infection, Healthline reported.

Identifying the cause of the foul odor could help reduce the foul odor. "For effective long-term odor control, it is important to reduce the amount of skin squames from entering the air-cooling units," study co-author Lai, Ka-man, who teaches biology at Hong Kong Baptist University, said in a recent statement. "To do so, the simplest way is to install an appropriate filter to capture the skin squames in the air."